A Wavelength Division Multiple Access Passive Optical Network (WDMA PON) is super high speed Internet transmission technology capable of replacing a current Very-High-Data-Rate Digital Subscriber Line (VDSL). If this WDMA PON is applied to a typical optical network, voice, data and video can be simultaneously used in real time for online games, super high speed Internet access, etc., in addition to voice and High Definition Television (HDTV) and various digital video services. In order to apply such a WDMA PON to a typical optical network, Fiber-To-The-Home (FTTH), allowing an optical fiber to be deployed to the home of a subscriber, must be realized.
In a WDMA PON using a conventional fixed-wavelength fabry-perot laser diode, a fabry-perot laser diode is deployed in an optical network terminator, and a Central Office (CO) injects incoherent light into the laser diode, so that the oscillation wavelength of the fabry-perot laser is fixed at the wavelength of the injected incoherent light. At this time, if the fabry-perot laser diode acts as a light modulator having a gain, a bias current approaches an oscillation threshold current. Therefore, a considerable amount of power is consumed by the fabry-perot laser diode. However, if a situation where power is not supplied to an optical network terminator, that is, a power failure situation, occurs, it is impossible to drive the fabry-perot laser diode, so that a situation where a subscriber cannot communicate may occur.
This situation is a great interruption when a current wired telephone line has been replaced. In the case of the current wired telephone line, since the central office supplies power through the telephone line, communication is possible even in a power failure situation. However, in the case of FTTH transmission technology including WDMA PON, when power is not supplied, as in a power failure, a communication channel cannot be provided. Therefore, a subscriber cannot communicate with others when an emergency situation occurs.
Generally, if a current above an oscillation threshold current is applied to a fabry-perot laser, there are multiple oscillation modes output. However, if incoherent light, in which a narrow band wavelength is filtered, is injected from the central office, the modes at the wavelengths differing from the wavelength of the injected light among the multiple oscillation modes are suppressed, and the only an oscillation mode having the wavelength identical to that of the light injected remains in the output. That is, only the oscillation mode having a wavelength identical to that of the injected light remains, thus enabling the fabry-perot laser diode having the miltiple oscillation modes to be operated as in a single mode. These are the principles of the WDMA PON using a fixed-wavelength fabry-perot laser. In this case, an incoherent light source is placed in a central office. Since the central office is not affected by a power failure, incoherent light is always injected into an optical network terminator from the central office. Therefore, if this incoherent light can be modulated and the modulated light can be returned to the central office, communication is possible even in a power failure.
The modulation of incoherent light injected at low power can be performed by applying a forward bias current below a threshold current or applying a reverse bias current to a fabry-perot laser. If the fabry-perot laser is modulated at a current below an oscillation threshold current, total light output power is reduced compared to the case where a current above a threshold current is applied, but a modulated optical signal with intensity sufficient to transmit a low speed signal, the speed of which is an integer times 64 kbps, such as a voice signal, can be generated. Therefore, since a current below a threshold current is applied, power consumption can greatly decrease compared to the case where a fabry-perot laser is driven using a current above a steady state threshold current. Therefore, a communication channel can be established for a considerable period of time using a small capacity battery at the time of a power failure. Further, in the case of a forward bias, the output power of a laser diode varies even when incoherent light is not externally injected, so that a fabry-perot laser diode can be operated by utilizing the forward bias characteristics even in a construction in which an incoherent light source does not exist.
In the meantime, if a reverse bias voltage is applied to a fabry-perot laser diode, the fabry-perot laser diode acts as an electro-absorption modulator that absorbs externally injected light, instead of acting as an amplifier that increases optical power. That is, the reverse-biased fabry-perot laser diode absorbs part of the incident light, and the remaining part thereof is output after reflected by a mirror. At this time, since absorptivity varies with an applied reverse bias voltage, the voltage is adjusted to convert an electrical signal into an optical signal. In this case, the front facet of the fabry-perot laser diode needs to be anti-reflection coated so as to perform efficient modulation. Such anti-reflection coating on the fabry-perot laser diode is adapted to reduce the extent of wideband light reflected from an incident side of the fabry-perot laser diode. That is, if a fabry-perot laser diode that is not coated is used, the amount of light reflected from the incident facet is greater than the amount of light obtained after being absorbed by the fabry-perot laser diode. Then, a central office has difficulty in distinguishing such modulated signals from the reflected incoherent light. Because a reverse current of a laser diode is much lower than a forward current, power consumption of the laser diode operated in this way can be greatly reduced compared to the case where the laser diode is operated in forward bias. Furthermore, in order to increase modulation efficiency, the incident side of the fabry-perot laser diode may be anti-reflection coated and/or an opposite surface may be high-reflection coated.